1. Field of the Invention
This invention relates generally to a fuel cell system that corrects cell voltage instability due to hydrogen starvation and, more particularly, to a fuel cell system including split sub-stacks that injects fresh hydrogen into a weak sub-stack before a reactive anode bleed is commanded in an effort to recover from a low cell voltage and improve system stability.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The MEAs are permeable and thus allow nitrogen in the air from the cathode side of the stack to permeate therethrough and collect in the anode side of the stack, referred to in the industry as nitrogen cross-over. Even though the anode side pressure may be higher than the cathode side pressure, the cathode side partial pressures will cause air to permeate through the membrane. Nitrogen in the anode side of the fuel cell stack dilutes the hydrogen such that if the nitrogen concentration increases beyond a certain percentage, such as 50%, the fuel cell stack becomes unstable and may fail. It is known in the art to provide a bleed valve at the anode exhaust gas output of the fuel cell stack to remove nitrogen from the anode side of the stack.
Some fuel cell systems employ anode flow-shifting where the fuel cell stack is split into sub-stacks and the anode reactant gas is flowed through the split sub-stacks in alternating directions. In these types of designs, a bleed manifold unit (BMU) is sometimes provided between the split sub-stacks that includes the valves for providing the anode exhaust gas bleed.
An algorithm may be employed to provide an online estimation of the nitrogen concentration in the anode exhaust gas during stack operation to know when to trigger the anode exhaust gas bleed. The algorithm may track the nitrogen concentration over time in the anode side of the stack based on the permeation rate from the cathode side to the anode side, and the periodic bleeds of the anode exhaust gas. When the algorithm calculates an increase in the nitrogen concentration above a predetermined threshold, for example 10%, it may trigger the bleed. This bleed, sometimes referred to as a proactive bleed, is typically performed for a duration that allows multiple stack anode volumes to be bled, thus reducing the nitrogen concentration below the threshold.
Another type of known anode exhaust gas bleed is known as a reactive bleed. In a reactive bleed, an algorithm calculates the fuel cell voltages and triggers a bleed when a stack cell voltage spread threshold is exceeded. Cell voltage spread is the difference between the maximum and minimum cell voltages of split sub-stack. The purpose of the reactive bleed is to reduce the cell spread due to cell starvation. This is typically due to excessive nitrogen accumulation or liquid water flooding in the flow fields in the anode side of the stack.
When a reactive bleed is commanded in a split sub-stack system, the system controller typically determines which bleed valve to open based on the current shift direction of the anode flow. In one known system, a saw tooth command signal is employed to determine which of the split sub-stacks is receiving hydrogen at any particular point in time. The saw tooth command signal is based on a range of values from 0 to 1, where if the saw tooth command signal is between 0 and 0.5, then hydrogen is sent to a first sub-stack and when the saw tooth command signal is between 0.5 and 1, the flow shift is reversed, and the hydrogen is sent to the second split sub-stack. During a bleed command, the bleed valve for the sub-stack that is down-stream to the sub-stack that is receiving the fresh hydrogen is opened, where the flow shift remains in this configuration until the bleed request is terminated. When the bleed request is terminated, the command signal is reset to 0 so that the first sub-stack is always the sub-stack that receives fresh hydrogen first after a bleed request has been terminated.
Two problems can be observed by this type of command for anode flow shifting and bleed requests. First, if a cell voltage spread of either of the split sub-stacks exceeds a spread threshold and a reactive bleed is commanded, the orientation of the flow shift may be such that the weak sub-stack having the greatest cell voltage spread may not be the one that is currently receiving hydrogen, and thus, will be the one from which the bleed occurs. In other words, if one of the sub-stacks has a low performing cell and that sub-stack is the down-stream sub-stack for the current flow shift direction, then the reactive bleed that would be commanded would inject fresh hydrogen into the other sub-stack and the bleed would be provided through the bleed valve at the output of the low performing sub-stack. Thus, the more stable of the two sub-stacks is the sub-stack that is receiving the fresh hydrogen during the bleed event, which would cause the voltage spread of the weak sub-stack to increase.
Further, after the bleed request is terminated, the saw tooth command signal is reset to 0 so that the same sub-stack is always the one that is receiving the hydrogen first. This causes the sub-stack that receives hydrogen by the saw tooth command signal during 0-0.5 to receive 50% more hydrogen that the other sub-stack. This situation can be illustrated as follows. Suppose the bleed request duration is τ and the saw tooth command signal period is T. In a worst case situation, as a result of the reset of the saw tooth command signal to 0 after a bleed request is terminated, the duration for the second sub-stack to receive hydrogen is τ+T/2 and the duration for the first sub-stack to receive hydrogen is τ+T. Therefore, the ratio of the duration for the each sub-stack receiving fresh hydrogen is given by:
                              a                      A            ⁢                                                  ⁢            to            ⁢                                                  ⁢            B                          =                                            τ              +              T                                      τ              +                              T                /                2                                              =                                                    τ                +                                  T                  /                  2                                +                                  T                  /                  2                                                            τ                +                                  T                  /                  2                                                      =                                          1                +                                                      T                    /                    2                                                        τ                    +                                          T                      /                      2                                                                                  =                              1                +                                  1                                                                                    2                        ⁢                                                                                                  ⁢                        τ                                            T                                        +                    1                                                                                                          (        1        )            
For low current density, the bleed request duration τ is usually small compared to the saw tooth command signal period T, and therefore, RAtoB is large. For example, for a stack current density j=0.1, the saw tooth command signal shift period T=6.09 seconds and the bleed period τ=3 s. Therefore:
                              R                      A            ⁢                                                  ⁢            to            ⁢                                                  ⁢            B                          =                              1            +                          1                                                                    2                    ⁢                                                                                  ⁢                    τ                                    T                                +                1                                              =                                    1              +                              1                                                                            2                      ×                      3                                        6.09                                    +                  1                                                      ≈            1.5                                              (        2        )            
This means that the first sub-stack tends to receive hydrogen 50% more often than the second sub-stack for the same bleed request condition. This calculation also explains that stack voltage drop occurs more often in low current density conditions and the second sub-stack tends to be the weak stack more often.